This is all I found
The exhaust gas recirculation (EGR) valve is vacuum controlled valve that regulates the amount
of exhaust gases that re-enter the cylinders. The powertrain control module (PCM) determines how
much based on engine load, temp, and other conditions. If the PCM detects that the amount of
exhaust gases entering the cylinder were insufficient or non-existent, this code is set.
Symptoms
No symptoms will likely be noticed by the driver other than the MIL (malfunction indicator lamp). However,
non visible symptoms will be increased combustion temperature and increased Nox emissions.
Causes
A code P0400 may mean that one or more of the following has happened:
Plugged EGR passage which restricts the flow of exhaust gasses
Faulty EGR solenoid
Faulty EGR solenoid wiring/harness
Vacuum lines damaged/disconnected to the EGR valve solenoid or to the EGR valve
Faulty EGR valve
Possible SolutionsSince EGR valve designs are different, no one test will suffice:
Using a scan tool, operate the EGR valve with the engine running. If the engine stumbles, the problem
was likely intermittent wiring problem or intermittent blockage problem
If engine doesn't stumble, operate the EGR valve manually, if possible. If engine doesn't stumble and die,
the ports are likely plugged. Removal of valve and cleaning of all ports will be required
Checking of solenoid can only usually be done with a scan tool, because most solenoids operate with a
duty cycle of voltage, rather than constant voltage
Check all vacuum lines, hoses, etc for any damage
Visually inspect the solenoid harness and the solenoid for damage
Replace EGR valve
